A thermoelectric conversion material has a wide range of application. While the greatest expectation is directed toward use as a clean energy source for converting thermal energy to electrical energy, various other applications have been researched to utilize its Peltier effect and put to practical as small-size cooling devices, heatsinks, high-temperature baths, electrotherms and others.
Thermoelectromotive force means a voltage V arising from the temperature difference ΔT between two contact points of a pair of different electric conductors jointed together, wherein the relation between the voltage V and the temperature difference ΔT may be expressed by the formula: V=αΔT. The coefficient α is referred to as “Seebeck coefficient”. In a process of converting thermal energy to electrical energy by utilizing the thermoelectromotive force, the effectiveness of the thermoelectric conversion material is represented by use of a figure of merit Z expressed by the formula: Z=α2σ/κ. A higher value of Z is indicative of superiority as the thermoelectric conversion material.
Among many previously reported or available thermoelectric conversion materials, Bi—Te based substances presently exhibits the highest figure of merit of about 3×10−3 (/K) but have a relatively small Seebeck coefficient of about 200 (μV/K) (“Practical Manual for New Material Technologies”, International Trade and Industry Documentary Research Inc., 1996, p 904).
Generally, rare-earth sulfides have a large Seebeck coefficient. In this context, it has been reported that sulfide based on either one of elements from La to Nd among lanthanide (III) (II) sulfides was irreversibly transformed from orthorhombic alpha-phase which is a low-temperature stable phase to tetragonal beta-phase and further to cubic Th3P4 type gamma-phase which is a high-temperature stable phase, and in particular, La2S3 and CeS3 were verified as thermoelectric conversion materials having Seebeck coefficients of +354 μv·deg−1 at 373 K and +574 μv·deg−1 at 373 K, respectively (G. V. Samsonov et al. “Sulfide Manual”, Japan-Soviet Information Service Inc., 1974, p 108).
It has also been reported that lanthanum sulfides La3-x S4 and La-A-S (wherein A is Ca or Ba) exhibited a maximum figure of merit 2.9×10−4(/K) (S. Katsuyama et al. “Journal of Thermoelectric Conversion Symposium '99”, 1999, p 56). However, the reported Seebeck coefficient is a maximum of about 100 (μv/K).
Further, the inventors have precedently made a presentation about research findings of Ce2S3 powder including its crystal structure, chemical analytical values, grain-size distribution and others (J. Am. Ceram. Soc., 81, 1998, p 145).